Contactless variable resistor



Jan- 31, 1967 R. J. WINTER 3,302,151

CONTACTLESS VARIABLE RES ISTOR /0 FIG. 2. /e

@g55/Q7" Ll M//N TER Jan 31', 1967 R. J. WINTER CONTACTLESS VARIABLERESISTOR Filed June 2l, 1965 2 Sheets-Sheet Z /S i INVENTOR. f' P05537d. Vl//NTER FIG. 6.

United States Patent O 3,302,151 CONTACTLESS VARIABLE RESISTOR Robert J.Winter, 12 Crestwood Drive, Avon, Conn. 06001 Filed June 21, 1965, ser.No. 465,351 6 Claims. (CI. 338-79) The invention hereafter disclosedpertains to variable resistors of the general type known aspotentiometers; and more particularly to a variable resistor Iutilizinga fliexible wire or wire-like resistance element but not utilizing orcomprising any sliding or brushing movable contact.

In the prior art there are disclosed variable resistors andpotentiometers of divers constructions, in which instruments theterminal connections are made to ends of a length of resistance wire orother resistance element and to a contact member whose position relativeto-or along the element is variable whereby to vary the potential orresistance exhibited between the contact terminal and one or the otherof the end terminal connections. All such potentiometers or variableresistors are characterized by a nonuniform variation of the electricalcontact-resistance evidenced between the Contact member and theresistance element as relative motion between the contact member and theelement occurs incident to adjustment being effected. Such variation ofcontact resist-ance, in the form of irregular variations of thepotential or resistance exhibited as noted, is in the trade designatedby the term contact noise. It is desirable to reduce such variation ofcontact-resistance or noise to the lowest possible values.

The present invention, by eliminating entirely the sliding or brushingcontact-member of a variable resistor or potentiometer, eliminates thesource of the abovementioned undesirable noisel According to theinvention, the two physical ends of a resistance element are permanentlyconnected to respective permanent rst and second terminals, and theresistance exhibited between the first and second terminals is varied byeffectively shortcircuiting a desired variable extent of the resistanceelement disposed between the first and second terminals and thus byvarying the extent of the resistance element not short-circuited. Thenoted result is accomplished by winding a desired extent of theresistance element from an insulation mandrel surface or form onto ahighly conductive mandrel surface or for-m, and vice versa; one end ofthe conductive mandrel surface or form, for example,

providing the second terminal, to which one .physical end of the elementis conductively secured, and a terminal connected to the other physicalend of the element, on the insulation mandrel surface or form, providingthe first terminal. Obviously, three terminals may be provided, ifdesired, by connecting the ends of the conductive mandrel surface torespective different terminals. However, since two of the threeterminals thus provided would be at the same potential, there willordinarily be no necessity for a third terminal. Preferably, and asherein illustrated in elementary exemplary form, the conductive mandrelsurface or form is stationary and of elongate form such as a cylinder,and the insulation mandrel surface is rotatable around the axis of theconductive stationary member surface and is prevented from rotatingabout its own axis as it is rotated about the conductive form. The twomandrels or forms are arranged with their axes parallel, and may presentuniformly helical winding formations or surfaces to receive theresistance element, or may, alternatively, present specially shapedWinding formations or surfaces whereby a desired functional relationshipis attained between rotation of the rotary mandrel surface and variationof exhibited resistance or po- 'tential drop.

ice

Since neither of the two aforementioned mandrels partakes of rotationabout its own axis, it is evident that no slip-ring or other sliding orbrushing electrical connection is required. Since the rotatable mandrelmerely rotates about the axis of the stationary mandrel but not on itsown axis, a iiexible electrical connection, such as a coil spring or aflexible wire connector, permanently connected at its one end to one endof the resistance element and at its other to a respective terminal,serves as a highly conductive 4path from the terminal to the end of theresistance element Iportion wound on the rotatable mandrel surface.

The preceding brief lgeneral description of the invention makes itevident that it is a prime object of the invention to provide a variableresistor of the wire or filamentelement type that is devoid of a slidingor brushing contact member.

Another object of the invention is to provide a variable resistoradapted to be adjusted by rotation of a rotary actuator and which istotally devoid of effects such as are produced by sliding or brushing ofa contact member on a resistance element.

Another object of the invention is to provide a rotary variable resistorin which all electrically active components are positively connectedtogether whereby electrical noise incident to sliding of electricalcontact means is completely obviated.

`Other objects and advantages of the invention are hereinafter set forthor made evident in the appended claims and the following detaileddescriptions of Ipreferred exemplary embodiments of variable resistorsaccording to the invention as illustrated in the accompanying drawingsforming a part of this specification.

In the drawings:

FIGURE 1 is a view in elevation, to no particular scale, showing anelementary preferred construction of variable resistor according to theinvention;

FIGURE 2 is a stepped or irregular sectional view of components of thevariable resistor depicted in FIGURE 1, viewed as indicated by thearrows and stepped broken line 2-2 in FIGURE 1;

FIGURE 3 is a transverse sectional view of structure depicted in FIGUREl, viewed as indicated 'by the arrows and broken line 3 3 in FIGURE l;

FIGUR-E 4 is a transverse sectional view of structure depicted in FIGURE1, viewed as indicated by the arrows and broken line 4-4 in FIGURE 1;

FIGURE 5 is a View in elevation, partly in section by removal of.portions of parts to reveal details, to no particular scale,illustrating a modified form of exemplary variable resistor structureaccording to the invention;

FIGURE 6 is a sectional view showing structure depicted in FIGURE 5,viewed as indicated by arrows and broken line 6-6; and

FIGURE 7 is a fragmentary sectional view taken as indicated by brokenline 7-7 of FIGURE S, with portions of structure brok-en away tofacilitate illustration of details.

Referring first to FIGURES 1 and 2, the preferred elementary exemplaryvariable resistor device, 10, `according to the invention comprisesprincipal supporting means in the form of a base 12 and upstanding posts12a and 12b which may conveniently be `formed integral with the base 12.Post 12a is arranged to support a stationary mandrel device 116 thefunctions of which are to provide a first and conductive winding surfaceupon which extents or lengths of a bare resistance element of wire-likeform may be wound, and to short-circuit the convolutions or turns of theresistance element thereon wound. As shown in the preferred form, base12 and post 12b are of metal, and mandrel device 16 is, as shown in itssimplest form, a

metal member affixed at one end to post 12b from which it is insulated'by an insulating bushing 14h and cooperating insulating lwasher 14W. Asis evident, if post 12b is formed of insul-ation, the bushing and washerare not essential; and similarly, the mandrel device may be in a formproviding conductive winding surface, as by a sleeve or wrapping ofmetal applied over an insulation bolt-like core, with an appropriateelectrical connection of the conductive jacket to a terminal. Othermodifications of structural features will be obvious in the light of thedisclosure, but in the interest of -brevity and clarity furtherdescription and explanation of the invention will be restricted to thesimpler illustrative structures depicted in the drawings. The end ofmandrel 16 next-adjacent to post 12b is used to support terminal meanswhich include, in the illustrated example, a terminal clip 16t andclamping nuts 16W engaged with a screw thread provided on the end of themandrel.

Rotatably mounted on the first or stationary mandrel device is acarriage device 18 whose function is to support a second elongatemandrel device with the axis of the latter generally parallel to that ofthe first mandrel device, and to rot-ate the second mandrel device aboutthe first while maintaining the axes generally parallel. The secondmandrel device provides an insulative (nonconductive) wrapping surfaceupon which extents of the bare resistance element may be wound. Therotary c-arriage device 18 comprises end members illustrated as --beingin the form of plates 18a and 18b both rotatably supported, as shown, onmandrel device 16. The carriage device further comprises means rigidlyconnecting together the plates 18a and 18b, the connecting means asshown being spacing bars 18e, 18d and 18e (FIGURE 3) affixed at the endsto respective ones of the plates. As shown, the spacing bars .are formedwith reduced ends that are fixed into complementary apertures or boresformed in the end plates. Rigid connection is effected by suitablemeans, such as by adhesive or by screwthreads in case the bars andplates are of insulation, as shown; or brazing or the like in case thosemembers are metallic.

The previously mentioned second mandrel device as shown in the preferredform in the drawings comprises an elongate mandrel 20 having an elongatenonconductive winding surface which preferably is grooved for receptionof turns of the resistance element 22. In the preferred form of theinvention the resistance element is a simple bare wire of resistivematerial. Mandrel 20 is provided with a portion of reduced diameter atone end, which portion is slid-able and rotatable in a radiallyextendingslot 118s (FIGURE 3) provided in plate 13a. Slot 18s is preferablyclosed, as it is when formed as a radially elongate aperture in plate18s. At its other end, mandrel 20 is arranged in similar manner to berotatable and radially slidable in a similar closed slot or apertureprovided in plate 18b. The rotary mandrel 20 is prevented from excessiveor undesired longitudinal shift or displacement in the carriage device18 by suitable restraining means such as insulation collars 20c and 20d(FIGURE 2) that are secured to the mandrel at respective opposite facesof plate 1819 and slidably engage respective faces of the plate.

The carriage device 18 is arranged to -be rotated about the axis ofmandrel 16 by application of driving effort or force to the extended end20e of mandrel device 20, by the bifurcated end of a crank or driver 24which is rotatably supported on and by a shaft 26 arranged in coaxialrelationship with mandrel device 16 and adapted to be turned or rotatedby any suitable driving means, such as, for example, a knob 28 affixedto the shaft. The shaft 26 is rotatable in bearing means here shown as abushing 30 that is fixed in an aperture in post 12a, as illustrated inFIGURES l and 2.

So that the flexible resistance element 22 may be wound off fixedmandrel device 16 onto rotary mandrel device 20, and vice versa,incident to rotation of carriage device 18, and to permit permanentnonbrushing electrical connection to that end of the resistance elementcarried by the rotary mandrel device 20, the latter must be restrictedor prevented from rotating relative to base 12 during rotation of thecarriage device 18. To the latter end or objective, rotation-restrictingmeans comprising an oscillatory governor '32 are provided. The governor32 as illustrated includes an upstanding pin 32p that is slidablyreceived in a transverse bore formed through collar 20d and mandreldevice 20, the pin being joined to, or formed as an integral upstandingextension of, an oscillatory arm 32a that is mounted for restrictedoscillation on a bearing surface provided by a bearing screw 32b (FIGURE1). The bearing screw is fixedly supported and is accommodated by atapped hole provided in a pedestal formed on base 12 as indicated inFIGURE l.

Thus as carriage device 18 is rotated, collar 20d is carried around theaxis of mandrel 16 and oscillates along the upstanding pin 32p and inturn moves pin 32p to and fro laterally of -base 12, oscillating arm 32ain the process. A slotted connection with bearing screw 32b permits thenecessary compound oscillation of arm 32a. As will be evident, whenmandrel device 20 is thus driven or carried around mandrel device 16,the resistance element will be unwound from the winding surface of oneof the two mandrel devices and concurrently `wound upon that of theother. The winding surfaces, here shown as grooves, must 4becomplementary whereby payoff and take-up of the element occurs withoutcreation of undue slack or tension in the element.

Permanent or nonbrushing electrical connection with that end of element22 that is secured to mandrel device 20 is effected by passing one endof the e-lement through a diagonal bore `formed through the mandreldevice (as indicated by dotted lines in FIGURES l and 2), andconductively and mechanically securing the element-end to the end of afiexible insulated conductor 34 (FIGURES 2 and 4) one end of which isanchored to collar 20d. The other end of conductor 34 is attached to aterminal 36 affixed to base 12 as indicated. Thus it is evident that asthe carriage device 1S is rotated, collar 20d is carried around acircular path orbiting shaft 26, but is prevented from rotating relativeto base 12 by governor pin 32p, and hence conductor 34 is merely flexedduring multipleturn rotation of the carriage device. Since the remote orother end of the resistance element is connected directly to theconductive surface of fixed -mandrel device 16, there exists acontinuous brushless electric path between terminal 36 and terminal 161,the electrical resistance of which path is varied by short-circuiting ofa greater or lesser extent of element 22 as the element is wound onto,or off of, the conductive surface of lmandrel device 16.

To accommodate slight variations or nonconformities in the windingsurfaces of the two mandrel devices, mandrel device 20 is permitted totravel radially in carriage device |18, to an extent restricted by theends of the radial slots in plates 18a and 18h in which the mandreldevice 20 is supported. To maintain the mandrel device 20 againstundesired radial travel and to keep taut the reach of the resistanceelement extending between the two mandrel devices, shaped wire springdevices 18p (FIGURE 3) and 18g (FIGURE l) are provided. The springdevices preferably are twins, and as depicted in FIGURE 3, are formed toeach have a free end portion pressing against mandrel device 20, anintermediate portion stressed around spacing bar 18e, and an anchoredend turned around bar 18d. Thus mandrel device 20 is resiliently pressedoutwardly in the slots in the end plates of the carriage, but may travelinwardly as may -be required. As is evident the spring constants of thesprings 18p and 18g `are such that a desired degree of tension iscreated and maintained in the resistance element 22.

A modified form of variable resistor according to the invention isillustrated in FIGURES 5, 6 and 7. Therein a stationary conductivemandrel l116 provides a tapered conductive winding surface of circularcross-section, preferably grooved as shown, and supported by andinsulated from an open rectangular frame 111 formed of flat striplikematerial. A rotary mandrel 120 is disposed with its axis parallel tothat of mandrel L16, and is provided with a tapered nonconductivewinding surface, also preferably grooved and of circular cross-sectionand with taper complementary to the taper of the stationary mandrel. Asindicated, the tapers are similarly directed, whereby the extent offlexible resistance element 122 that is unwound Ifrom either of themandrels during any rotation of the carriage is equal to that wound ontothe other mandrel. Fixed mandrel 116 is provided with terminalconnection means as shown and in fashion similar to the previouslydescribed embodiment of apparatus.

Rotary mandrel 120 is arranged to be driven about mandrel 116 in bothdirections by being carried at one of its ends in a slotted crank arm124, and by being carried at its other end in a radially slidable cup118C disposed in a slot in a rotary member 118 which is held captive onbut permitted rotation around a bearing member 116b disposed on theanchored end of mandrel 116, as depicted. The end of mandrel 120 isrotatable in the guide cup 118C, and the cup is spring-urged outwardlylby a leaf spring 118s afiixed at one of its ends to the member 118 asindicated in FIGURE 7. At its driven end mandrel 120 is affixed in ashaped collar 120e, and extends through a radially elongate aperture incrank arm 124 in which it is held captive by nut and washer means asindicated in FIGURE 5. The collar 120e is stressed or outwardly urged bymeans including a leaf-spring 124s as indicated in FIGURE 6, so as tocooperate with spring 118s to maintain under tension the reach ofresistance element 122 between the two mandrels.

Crank arm 124 is aliixed to theinterior end of a rotary driver shaft 126that is journaled in a bushing |130 that is mounted in an accommodatingaperture in frame 111, as indicated in FIGURE 5. Shaft 126 is retainedin proper position fby retainer rings 126r disposed in accommodatingvannular grooves in the shaft, at respective ends of bushing 130. Anysuitable controllable source of torque may be used to rotate shaft 116and thereby cause short-circuiting of more or less of element '122 bythe conductive winding surface of mandrel device 116; and as shown, amanually-operable knob 128 is shown as exemplary means for the indicatedpurpose.

Rotation of mandrel 120 relative to frame |111 and base 112 to which theframe is fixedly mounted, is prevented by means including an oscillatorygovernor which comprises a pin 13211 which is slidably received inaligned bores extending through collar 120C and mandrel 120 as indicatedin FIGURES 5 and 6, pin 13211- being integral with or secured to anoscillatory arm 132a suitably secured to frame 111 by means including apivot bearing 132b. Arm 132e is slotted for compound oscillation onbearing 132b.

Electrical connection to the end of element 122 carried on movablemandrel 120 is by way of an anchor screw =120s secured in insulationcollar 120C and to which one end of element 122 is secured after beingdrawn through a transverse -anchoring bore formed in the mandrel, asshown in FIGURES 5 and 6. A flexible insulated conductor 134 extendsbetween anchor screw 120s and an insulated terminal 136 afiixed on base112 as indicated in FIGURE 6. Electrical operation of the modified formof variable resistor depicted in FIGURES 5, 6 and 7 is substantially thesame as that of the principal embodiment of the invention, except thatdue to the speciallyshaped mandrels, the variation of resistance perdegree of revolution of the driving means is made to follow a nonlinearmathematical relationship. As is evident, the taper of the mandrelsmaybe -varied to provide many other relationships.

While very elementary constructions of the variable resistor have beenillustrated and described so as to more easilyand simply portray theprinciples of the invention, it will be evident to those skilled in theart that refinements and modifications may be made, all within the truespirit and scope of the invention. Accordingly it is not desired thatthe invention be restricted to the details of the illustrated exemplaryelementary forms, but only as defined and restricted by the appendedclaims.

I claim:

1. A variable resistor device comprising:

first means, including supporting means and an elongate fixed mandreldevice fixedly supported thereon, said fixed mandrel device defining afirst axis and having a conductive surface providing a first windingsurface upon which a flexible conductor may be wound in numerous turnsor convolutions;

second means, including a carriage supported by said first means forrotation about the said first axis, and means for causing rotation ofsaid carriage about said first axis through numerous revolutions;

third means, including an elongate rotary mandrel device rotatablysupported -by said carriage for rotation about a second axis parallel tosaid first axis, said Irotary mandrel device having an insulated surfaceproviding a second winding surface complementary to said first windingsurface and upon which a flexible conductor may be wound;

fourth means, including means connected to said first means and t-o saidrotary mandrel device, for preventing rotation of said rotary mandreldevice relative to said supporting means and permitting rotation of saidrotary mandrel device relative to said carriage incident to -rotation ofsaid carriage about said first axis;

and fifth mean-s, including an elongate bare flexible resistive elementwound in part upon said conductive first winding surface and fixedlyconnected thereto, and in part wound upon said insulated second windingsurface and adapted to be wound from either of said Winding surfaces tothe other incident to rotation of said carriage, said fifth meansfurther including a first fixed electric terminal means fixedlyconnected to said first winding surface, and a second fixed electricterminal means and a flexible insulated conductor fixedly connected atone end to said flexible resistive element wound on said rotary mandreldevice;

whereby the effective electrical resistance exhibited between said firstand second fixed electric terminal means may be varied without brushingor wiping contact and by rotation of said carriage.

2. A resistor device as defined in claim 1, in which said first windingsurface is provided by a groove disposed along a longitudinally curvedsurface of substantially cylindrical cross-section whereby increments ofthe length of the groove bear a predetermined nonlinear mathematicalrelationship to degrees of rotation of said carriage, and in which saidsecond winding surface is comp-lementary to said first winding surfacewhereby to take up and pay off said flexible resistance as required toproperly accommodate the latter incident to rotation of the carriage,whereby variation of resistance exhibited between said terminals variesin accord with a predetermined nonlinear mathematical relationshiprelative to degrees of rotation of said carriage incident t-o rotationof said carriage.

3. A resistor device as defined in claim 1, in which said founth meansincludes an oscillatory member pivoted to said first means and having arectilinear arm extending through a transverse bore formed through saidrotary mandrel device, thereby preventing notation of said rotarymandrel device relative to said first means while permitting rotation ofsaid carriage and second mandrel device around said first axis to causeunwinding and winding of said resistive element relative to said firstand second mandrel devices whereby to vary the extent of said resistiveelement short-circuited by said conductive winding surface to vary theelectrical resistance exhibited between said terminal means.

4. A variable resistor device as defined in claim 1, in which saidcarriage device includes first and second end members rotatable on saidthird meatns and spacing bar means rigidly connecting together the endmembers, and in which said end members are provided with slots carryingrespective end portions of said rotary mandrel device to permit increaseand decrease of spacing between the fixed and rotary mandrel devices,and resilient means tending to move the rotary mandrel away from lthefixed mandrel.

5. A variable resistor device adapted to exhibit between iixed terminalsthereof any finite value of resistance between determined minimum andmaximum values without movement of a contact on a resistance element,said resistor device comprising:

first means, including supporting means and xed rterminals supportedthereon;

second means, including an elongate bare iiexible resistance element;

third means, including a xed mandrel with a conductive winding surfacethereon, and a second mandrel with an insulated surface Ithereon, andmeans for relatively rotating one of said mandrels around the other andfor preventing rotation of either of the mandrels relative to saidsupporting means, whereby to wind portions of said resistance fromeither of said mandrels onto the other; and

fourth means, including means whereby one of said terminals ispositively connected to said conductive winding surface, and meansincluding a flexible in- Sula-ted conductor interconnecting the other ofsaid terminals and a portion of said resistance element wound upon saidsecond mandrel;

whereby incident to relative rotation of said mandrels portions of saidresistance element are transferred by winding onto one `of said mandrelsfrom the other thereof to change the extent of said element wound uponsaid conductive winding surface to thereby change the resistanceexhibited between said terminals.

6. A variable resistor device adapted .to exhibit between fixedterminals thereof any finite value of resistance between a determinedhighest resistance value and a determined lowest resistance value,without interruption of a continuous electrical path between theterminals and without movement of a contact on a resistance element,

said variable resistor device comprising:

supporting means, .and stationary electric terminals `supported by thesupporting means;

elongate stationary means iixedly supported by said supporting means andpresenting a Winding surface, said elongate stationary means defining anelongate axls;

elongate rotary means having a winding surface and an elongate axisextending generally in the same direction as that of said elongatestationary means;

means fior rotating said rotary means;

' an elongate flexible bare resistance element having |one end xedlyconnected to a rst lone of said terminals and wound in part on each ofsaid winding surfaces;

one of said elongate stationary and elongate rotary means having ahighly conductive winding surface effective to short-circuit turns ofsaid resistance element wlound thereon; and

flexible conductor means connecting the sec-ond yone of said terminalsto the end of the portion of said resistance element wound on saidwinding surface of said elongate rotary means,

whereby upon rotation of said rotary means an extent of said resistanceelement is unwound from one of said winding surfaces to the otherWinding surface and the extent of said resis-tance elementshort-circuited by said conductive winding surface is varied and theresistance exhibited between said ter-minals is varied.

References Cited by the Examiner UNITED STATES PATENTS 2,662,150 12/1953Mairs 338-79 2,666,905 1/1954 Gerlach 336-15 2,683,202 7/1954- Brown338--79 2,921,283 l/l960 Luther 338-79 RICHARD M. WOOD, PrimaryExaminer.

W. D. BROOKS, Assistant Examiner.

1. A VARIABLE RESISTOR DEVICE COMPRISING: FIRST MEANS, INCLUDINGSUPPORTING MEANS AND AN ELONGATE FIXED MANDREL DEVICE FIXEDLY SUPPORTEDTHEREON, SAID FIXED MANDREL DEVICE DEFINING A FIRST AXIS AND HAVING ACONDUCTIVE SURFACE PROVIDING A FIRST WINDING SURFACE UPON WHICH AFLEXIBLE CONDUCTOR MAY BE WOUND IN NUMEROUS TURNS OR CONVOLUTIONS;SECOND MEANS, INCLUDING A CARRIAGE SUPPORTED BY SAID FIRST MEANS FORROTATION ABOUT THE SAID FIRST AXIS, AND MEANS FOR CAUSING ROTATION OFSAID CARRIAGE ABOUT SAID FIRST AXIS THROUGH NUMEROUS REVOLUTIONS; THIRDMEANS, INCLUDING AN ELONGATE ROTARY MANDREL DEVICE ROTATABLY SUPPORTEDBY SAID CARRIAGE FOR ROTATION ABOUT A SECOND AXIS PARALLEL TO SAID FIRSTAXIS, SAID ROTARY MANDREL DEVICE HAVING AN INSULATED SURFACE PROVIDING ASECOND WINDING SURFACE COMPLEMENTARY TO SAID FIRST WINDING SURFACE ANDUPON WHICH A FLEXIBLE CONDUCTOR MAY BE WOUND; FOURTH MEANS, INCLUDINGMEANS CONNECTED TO SAID FIRST MEANS AND TO SAID ROTARY MANDREL DEVICE,FOR PREVENTING ROTATION OF SAID ROTARY MANDREL DEVICE RELATIVE TO SAIDSUPPORTING MEANS AND PERMITTING ROTATION OF SAID ROTARY MANDREL DEVICERELATIVE TO SAID CARRIAGE INCIDENT TO ROTATION OF SAID CARRIAGE ABOUTSAID FIRST AXIS; AND FIFTH MEANS, INCLUDING AN ELONGATE BARE FLEXIBLERESISTIVE ELEMENT WOUND IN PART UPON SAID CONDUCTIVE FIRST WINDINGSURFACE AND FIXEDLY CONNECTED THERETO, AND IN PART WOUND UPON SAIDINSULATED SECOND WINDING SURFACE AND ADAPTED TO BE WOUND FROM EITHER OFSAID WINDING SURFACES TO THE OTHER INCIDENT TO ROTATION OF SAIDCARRIAGE, SAID FIFTH MEANS FURTHER INCLUDING A FIRST FIXED ELECTRICTERMINAL MEANS FIXEDLY CONNECTED TO SAID FIRST WINDING SURFACE, AND ASECOND FIXED ELECTRIC TERMINAL MEANS AND A FLEXIBLE INSULATED CONDUCTORFIXEDLY CONNECTED AT ONE END TO SAID FLEXIBLE RESISTIVE ELEMENT WOUND ONSAID ROTARY MANDREL DEVICE; WHEREBY THE EFFECTIVE ELECTRICAL RESISTANCEEXHIBITED BETWEEN SAID FIRST AND SECOND FIXED ELECTRIC TERMINAL MEANSMAY BE VARIED WITHOUT BRUSHING OR WIPING CONTACT AND BY ROTATION OF SAIDCARRIAGE.